feat: change default macOS data path to ~/.skill-manager

[execsumo]

This PR updates the default macOS application directory from ~/Library/Application Support/skill-manager to ~/.skill-manager. It includes a fallback mechanism to automatically continue using the legacy Application Support path if it already exists, ensuring backwards compatibility for existing users.

[chzh12]

Thanks for the PR. I’m not comfortable approving this as a normal implementation change without an explicit product/distribution decision.

Changing the default macOS app-owned data directory from ~/Library/Application Support/skill-manager to ~/.skill-manager changes where new users store shared skills, MCP manifests, scan configs, settings, runtime state, and logs. Even with the legacy fallback, this is a user-visible storage and support-policy change rather than a small refactor.

Please get explicit agreement on the macOS default data path first, including migration/support expectations and release-note impact, before we merge this.